In many applications, it is necessary to distinguish an original article from a copy or counterfeit to validate the original article. An original article that includes an authenticating feature can be validated in many ways. Some methods involve visible (i.e., overt) authenticating features that are disposed on or incorporated into the article, such as a hologram on a credit card, an embossed image or watermark on a bank note, a security foil, a security ribbon, colored threads or colored fibers within a bank note, or a floating and/or sinking image on a passport. While these features are easy to detect with the eye and may not require equipment for authentication, these overt features are easily identified by a would-be forger and/or counterfeiter. As such, in addition to overt features, hidden (i.e. covert) features may be incorporated in original articles. Examples of covert features include invisible fluorescent fibers, chemically sensitive stains, and taggants such as luminescent pigments or fluorescent dyes that are incorporated into the substrate of the article.
While authentication of covert features that employ taggants is highly reliable through use of authentication equipment, the cost of equipment required for authentication is generally too high for and/or unavailable to the typical consumer or small business owner. Further, most authentication equipment is bulky, rendering use thereof inconvenient for many applications. Production of small, inexpensive authentication devices is challenging because the authentication devices generally include a significant amount of hardware, including an exciting light source, a photodetector, a gain amplifier, and perhaps other components, and it is difficult to include all of those components in a sufficiently small package. To the extent that small authentication devices are available, the small size of the authentication devices makes the authentication devices susceptible to various environmental factors that render accurate authentication difficult. For example, small authentication devices generally include the exciting light source and the photodetector in such close proximity that light produced by the exciting light source enters the photodetector via strong scatter from an article under interrogation. Intensity of the light from the exciting light source is generally many orders of magnitude higher than intensity of emissions from the taggants. While optical filters are generally employed so that the photodetector is not saturated with too much light from the exciting light source, it is difficult to effectively block light produced directly by the exciting light source while still allowing passage of emitted radiation from the taggants. Many excitation sources such as LEDs exhibit emission very far from their primary emission spectral range and that emission can be significant and within the passbands of the optical filters. While it is possible to further block light produced by the exciting light source by employing a thin film dielectric stack filter such as a band pass or a long pass filter, there may still be some undesired level of leakage that remains within the desired pass band of the filter system that is not related to the emitted radiation from the taggants. A number of systems would essentially be fooled by the large amount of signal within the required detection spectral range. Further, many taggants exhibit an overlap in emission wavelength with light produced by the exciting light source such that the overlapping wavelengths cannot be blocked while still enabling detection of the emissions from the taggant.
Accordingly, it is desirable to provide authentication systems, authenticating devices, and methods of authenticating value articles that are capable of effectively interrogating value articles that include a luminescent material for emissions from the luminescent materials while minimizing the impact of light produced by the exciting light source on authentication, even when light from the exciting light source passes into the photodetector. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.